The objective of this proposal is to understand the molecular mechanisms involved in both the expression and transposition of the maize Suppressor- mutator (Spm) transposable element, as well as in its developmental regulation. The first objective is to identify the Spm-encoded gene products required for transposition and positive autoregulation. An in vivo transposition assay has been developed using transgenic tobacco and will be used to assess the effects of in vitro frameshift, deletion, and insertion mutations on the ability of Spm to trans-activate transposition. New in vitro and in vivo assays will be developed to analyze the element's genetically identified autoregulatory functions, as well as to study the interactions between the proteins required for element transposition and regulation and their target sequences within the element. Experiments will be done to further understand the genetic mechanism that underlies the stable, but reversible inactivation of the element, which is associated with changes in the methylation of cytosine residues near the Spm element's transcription start site. Efforts will be made not only to identify the methylating enzymes, but also to understand the biochemical basis of the observed decrease in methylation associated with the genetic activation of an inactive element. Finally, genetic and molecular experiments will be carried out to determine the relationship between the genetic mechanism underlying the developmental programming of Spm expression and other basic plant developmental determination events.